Electrocoating process

ABSTRACT

A METHOD FOR IMPROVING AND MAINTAINING THE QUALITY OF THE ELECTROCATING DEPOSITED ON A METALLIC ANODE WHICH COMPRISES REMOVING INORGANIC ACID ANIONS FROM THE ELECTROCOATING BATH.

Unimd tates Patent 3,591,478 ELECTROCOATING PROCESS James R. Erickson,Parma, Ohio, assignor to SCM Corporation, Cleveland, Dhio No Drawing.Filed May 6, 1968, Ser. No. 727,009 Int. Cl. Btllk 5/02; (323i) 13/00US. Cl. 204181 7 Claims ABSTRACT OF THE DICLOSURE A method for improvingand maintaining the quality of the electrocoating deposited on ametallic anode which comprises removing inorganic acid anions from theelectrocoating bath.

This invention relates to an improved method of continuouslyelectrocoating metallic substrates wherein the quality of theelectrically deposited film is maintained through the removal ofinterfering inorganic, acid anions from the electrocoating bath.

In one aspect, this invention relates to an improved method ofelectroplating a series of aluminum articles, in a single bath, with anionized, multicomponent paint.

Electrocoating is the electrodeposition of a resinous material (with orwithout pigments) on surfaces in an electrical circuit, from ionic,Water dispersions of these resins by means of an electrical current. Inthe electrocoating process, the object being coated is the anode, andthe cathode is usually the tank containing the aqueous dispersion ofelectropaint. The anode and cathode are electrically connected to anexternal power source to complete the circuit.

In the electrocoating process a coating bath is formed by dispersing, inan aqueous medium, a synthetic organic resin having a plurality of waterionizable, anionic, functional groups within its molecular structurewith a water ionizable dispersal assistant for such resin. The articleto be coated is immersed in this coating bath while the article isconnected as the anode in an electrical circuit. The tank containing thebath serves as the cathode in the circuit (alternatively, immersioncathodes can be used) and a direct current potential (or its electricalequivalent) is impressed across these electrodes. As the electricalcurrent flows between the electrodes, the ionized, anionic dispersedresin is electrically deposited on the anode, and is converted into anessentially water insoluble coating thereon. The coating on the anode isthen thermally cured if required.

This electrocoating is done with direct (unidirectional) current. Inmost cases, such current ordinarily is rectified AC current having abouta 5 to 15% ripple factor. However, the current can have a greater orlesser ripple factor, or even can be half wave rectified alternatingcurrent, and so on, providing the net effect is unidirectional and thus,the current is direct current.

Useful voltages across the bath can be as low as 15 or even lower, andshould not be so high as to burn through the deposited coating.Practical maxi mum deposition voltages are 350-500 volts for manyresinous systems, although higher voltages can be used with selectedsystems, particularly if the duration of the higher voltage period isvery short.

The electropaint is, of course, electrolytic in nature and completes thecircuit between the anode and the cathode. This electropaint dispersioncomprises a film forming paint binder containing a polycarboxylic acidresin at least partially neutralized with a water soluble aminocompound. The polycarboxylic acid resin usually has an electricalequivalent weight between about 500 and about 20,000, and an acid numberbetween about 20 and about 300. The polycarboxylic resin exhibits ani-3,591,478 Patented July 6, 1971 Too onic polyelectrolyte behavior asindicated by the deposition on the anode substantially directlyproportional With the electric current being passed through the bath.

The film-forming material employed in an electrocoating process of thetype herein contemplated can constitute the sole coating material withinthe bath or it may include or be employed with pigments, metallicparticles, dyes, drying oils, extenders, etc., and may be dispersed as acolloid, emulsion, emulsoid or apparent solution. The primary orbackbone resin or resins employed in preparing the film-forming bindermay include, but not by way of limitation, alkyd resins, acrylateresins, epoxy resins, phenol-formaldehyde resins, hydrocarbon resins,and other organic resins or mixtures of one or more of the foregoingresins with another of the resins heretofore mentioned or with otherfilm-forming organic materials including binding agents and extendersconventionally employed with paints. Such materials may be reacted withor accompanied by other organic monomers and/or polymers including, butnot by way of limitation, hydrocarbons and oxygen substitutedhydrocarbons such as ethylene glycol, glycerol, monohydric alcohols,carboxylic acids, ethers, aldehydes and ketones.

Since the binder material is to be deposited anodically the resin willhave free or Water dissociable carboxyl groups or their equivalentwithin the molecular structure of the resin. These can be the result ofthe original formulation of the resin or subsequently introduced bychemically reacting a suitable resin with monomers and/ or polymerswhich introduce such groups into the binder to be used in coating.Film-forming materials that have been found to be particularly suitablefor anodic deposition include, but not by way of limitation, coupledsiccative oils, e.g. coupled glyceride drying or semidrying oils such aslinseed, sunflower, safilower, perilla, hempseed, walnut seed,dehydrated castor oil, rapeseed, tomato seed, menhaden, corn, tung,soya, oiticica, or the like, the olefinic double bonds in the oil beingconjugated or nonconjugated or a mixture, the coupling agent being anacrylic olefinic acid or anhydride, preferably maleic anhydride, butalso crotonic acid, citraconic acid, or anhydride, fumaric acid, or anacrylic olefinic aldehyde or ester of an acyclic olefinic ester such asacrolein, vinyl acetate, methyl maleate, etc., or even a polybasic acidsuch as phthalic or succinic, particularly coupled glyceride oils thatare further reacted with about 225% of a polymerizable vinyl monomer;maleinized unsaturated fatty acids; maleinized rosin acids, alkydresins, e.g. the esterification products of a polyol with a polybasicacid, particularly glyceride drying oil-extended alkyd resins; acidichydrocarbon drying oil polymers such as those made from maleinizedcopolymers of butadiene and diisobutylene; diphenolic acid and likepolymer resins; and acrylic and vinyl polymers and copolymers havingcarboxylic acid groups such as butyl acrylate-methylmethacrylate-methacrylic acid copolymers, vinyl acetate-acrylic acidcopolymers, acrylic acid and lower alkyl (C substituted acrylicacid-containing polymers, i.e. those having carboxyl groups contributedby alpha, beta unsaturated carboxylic acids or residues of these acids,etc. Dispersion of these polycarboxylic acid resins in water is assistedby the addition of a suitable basic material such as ammonia, watersoluble amines, mixtures of monomeric and polymeric amines, KOH etc. ThepH of the bath is, of course, dependent upon the relative concentrationsof acidic and basic materials therein.

The acid number of resins without appreciable content of anhydridegroups can be determined with KOH by the ASTM standard method 55554. Ifappreciable anhydride groups are present, the acid number can bedetermined by refluxing a 1.5-2 gram sample of the portion of the resinfor one hour with 50 ml. of 0.5 N aqueous KOH and 25 ml. of pyridine,then back titrating with 0.5 N HCl to a phenolphthalein end point.

The electrical equivalent weight of a given resin or resin mixture isherein defined as that amount of resin or resin mixture that willdeposit per Faraday of electrical energy input under the conditions ofoperation set forth in detail in the succeeding paragraph. For thispurpose the value of one Faraday in coulombs is herein taken to be107.88 (atomic weight of silver) +0.001118 (grams of silver deposited byone coulomb from silver nitrate solution) or 96,493. Thus, if 0.015 gramof coating, the binder polycarboxylic acid resin moiety of which is 90%by weight and the balance of which is amino compound used to disperse itin the bath is transferred and coated on the anode per coulomb input tothe process, the electrical equivalent weight of the resin is about 1303or 0.0l l07.88+0.001118.

By way of further illustration, the electrical equivalent weight (in thenature of a gram equivalent weight in accordance with Faradays laws) ofa particular polycarboxylic acid resin or resin mixture is simply andconveniently found for typical process conditions standardized on asfollows: a polycarboxylic acid resin concentrate is made up at 65.56 C.(150 F.) by thoroughly mixing 50 grams of the polycarboxylic acid resin,8 grams of distilled water and diisopropanolamine in an amountsulficient to yield resin dispersion pH of 7.8 or slightly lower afterthe concentrate has been reduced to 5% by weight resin concentrationwith additional distilled water. The concentrate is then diluted to oneliter with additional distilled water to give 5% resin concentration inthe resulting dispersion. (If a slight insufiiclency of the amine hasbeen used, and the dispersion pH is below 7.8, pH is brought up to 7.8with additional diisopropanolamine). The dispersion is poured into ametal tank, the broadest walls of which are substantially parallel withand 2.54 cm. out from the faces of a thin metal panel anode. The tank iswired as a direct current cathode, and the direct current anode is agauge, 10.17 cm. (4 inches) wide, tared steel panel immersed in the bath7. 62 cm. (3.5 inches) deep. At 26.67 C. (80 F.) bath temperature directcurrent is impressed from anode to cathode at 100 volts for one minutefrom an external power source, the current measured by use of acoulometer, and the current turned off. The anode panel is removedimmediately, rinsed with distilled water, baked for 20 minutes at 176.67C. (350 F.) and weighed. All volatile material such as water and amineis presumed to be removed from the film for practical purposes by thebaking operation.

The difference between tared weight of the fresh panel and final weightof the baked panel divided by the coulombs of current used, times107.88, divided by 0.001118 gives the electrical equivalent weight ofthe resin for purposes of this invention.

To avoid duplication, the method of this invention is explained infurther detail using for purposes of illustration that embodiment ofelectropainting which has proven most practical to date in the light ofthe present state of this techniology, i.e. electropainting wherein atleast a substantial portion of the film-forming resinous material is asynthetic polycarboxylic acid resin and is employed with a dispersalassistant comprising a water soluble amino compound.

The especially suitable water soluble amino compounds are soluble inwater at 20 C. to the extent of at least about 1% basis weight ofsolution and include hydroxy amines, polyamines and monoamines such as:monoethanolamine, diethanolamine, triethanolamine, N-rnethylethanolamine, N-aminoethyl ethanolamine, N-methyl diethanolamine,monoisopropanolamine, diisopropanolamine, triisopropanolamine,polyglycol amines such as HO(C H O) C H NH hydroxylamine, butanolamine,hexanolamine, methyldiethanolamine, octanolamine, and alkylene oxidereaction products of monoand polyamines such as the reaction product ofethylene diamine with ethylene oxide or propylene oxide, laurylaminewith ethylene oxide, etc.; ethylene diamine, diethylene triamine,triethylene tetramine, hexamethylene tetramine, tetraethylene pentamine,propylene diamine, 1,3-diaminopropane, imino-bispropyl amine, and thelike; and mono-, di-, and trilower alkyl (C amines such as mono-, di-,and triethyl amine.

The best films are deposited when about 30-60% of the total aminoequivalents present in the bath, both combined and free, are contributedby water soluble polyamine, and thus it is preferred to operate thatway. Preferably, it is diethylene triamine for efficiency and economy.The polyamine can be added to the bath along with supplemental binderconcentrate composition dosing or separately.

The hydroxy amines, particularly those that are allphatic in nature atpoints of hydroxyl attachment, such as the alkanol amines, are also veryuseful for treating the polycarboxylic acid resin for dispersion andappear to have some desirable resin solubilizing effect in water overand above their neutralizing action. As a practical matter, the watersoluble amino compound present in the bath over and above that amountnecessary to impart anionic polyelectrolyte behavior to the particularpolycarboxylic acid resin in the binder can be considered excess and isdesirable, providing that the bath pH does not get so high that the bathabsorbs CO from atmosphere, or the high amine concentration lowers thebath resistance to below about 500 ohm-centimeters. Broadly, theproportion of amine used can be between about 2 and about 7 times, andpreferably between about 3.5 and about 5.5 times, the minimum amountnecessary for imparting anionic polyelectrolyte behavior to theparticular binder resin or resin mixture in the bath. Specificresistance of the bath as made up is advantageously between about 700and about 1000 ohm-centimeters to deposit coatings about 25 micronsthick as a priming coat. Higher bath resistance gives a thinner film andvice-versa.

Ammonia alone can be used but is less advantageous in my process forpartially neutralizing the acid resin or resin mixture because it is sohighly volatile at operating temperatures, small additions of it cancause comparatively large changes in pH of the bath, and baths using ittend to pick up CO from the air easily and thus are susceptible touncontrolled change in electrical characteristics. Accordingly, ammoniais used only to assist in dispersing the resin in the bath along withother water soluble amino compounds, and not to the exclusion of otherwater soluble amino compounds.

When the polycarboxylic acid resin binder in the bath is substantiallybelow about 1%, the film deposition is not as good as at higherconcentrations. At even lower resin concentrations in the bath theevenness, smoothness, adhesion and thickness of the film deterioratesextremely rapidly. When the resin dispersion concentration issubstantially above about -40% by weight, the bath viscosity can becomeso high that there is paint dragging when the coated body is withdrawnfrom the bath, that is, paint adheres and flows off non-uniformly togive an uneven deposit. The upper practical limiting concentration, itshould be understood, will be to some extent a function of theparticular resin in the bath at operating temperature (e.g. about 1550C. generally) correlative to its ease of dispersion of dissolution inwater, its electrical equivalent weight, and its specific rate of changeof viscosity with dispersion concentration. The 3540% represents apractical maximum.

Also, the bath viscosity is especially important in large scaleoperations where electrical energy converted to bath heat has arelatively small area per unit volume of bath container to dissipatefrom. Accordingly, as viscosity goes up, the efficiency of heat transferwith cooling devices internal or external to the bath and from the tankwalls themselves decreases substantially. Handling of the fluid in thebath and its drainage from the coated articles as they are withdrawnalso are distinctly inferior when the viscosity of the bath risesgreatly above that of water, i.e. more than about 200 times that ofwater. Heat control in the bath is important within a temperature rangeof roughly 15 to 50 C. to prevent the generation of undesirablevolatile. materials and even the destabilizing or undue additionalpolymerization of the paint dispersions in some cases. With a bathviscosity not above about 30 times that of water the heat control can bevery simple since the efficiency of heat transfer is quite high.

The proportion of amino compound, particularly hydroxy amines in thebath, can be used to manipulate bath viscosity, the higher proportionsgenerally promoting apparent solubilization of the resin and somereduction in viscosity.

In the exemplary paint baths described hereafter, the resin in the bathdispersions shows anionic polyelectrolyte behavior because deposition ofthe resin on the anode is essentially directly proportional with thedirect current passing through the bath. The quotient of coulombs ofelectricity per gram of a particular resin binder deposited is virtuallyindependent of voltage in the operating range (less than about 5-10%variation), when allowance is made for the additional current used todrive the varying concentrations of amino compound to the cathode, evenwhen the maximum voltage is doubled or trebled in the operating range of100-500 volts. It further appears that when the polyelectrolyte resinbinder coats tenaciously on a pigment or other particle in the bath,such particle assumes the migration properties to the anode similar tothe polycarboxylic acid resin itself.

The polycarboxylic acid resins in the bath appear to exhibit theelectrical migration property of anionic solutes, the resin ion presentcapable of being thought of as [R(COO)n] having n negative charges(where R represents the resin nucleus and COO represents a carboxylradical). For illustration the amino ions resulting from neutralizingthe resin in the bath (where the water soluble amine is used, is forexample, a primary monoamine) can be thought of as [R'NH Where R'represents the amino compound nucleus.

Direct current is passed through the bath at a potential gradient ofabout 50 to 1000 volts to deposit a coating film of the anioniccarboxylic resin on the anode workpiece.

As the coating builds up on the anode, the electrical resistance of thecircuit increases, thus concentrating the electrical energy on coveringpinholes, and other inacces sible areas until a coating of uniformresistance (and uniform thickness) is obtained. The coated workpiece isthen removed from the bath and the coating is cured at elevatedtemperatures according to conventional practices.

One of the primary advantages of this process is that a large number ofworkpieces can be coated in a short period of time using a single tankof electropaint dispersion.

Accordingly, once the tank has been filled with the resin dispersion,the coating operation can be continually conducted over prolongedperiods (i.e., weeks or months) without dumping, cleaning and refillingthe tank. It is, of course, necessary to replenish the tank with theanionic, polycarboxylic resin, as this resin is removed from the bath inthe form of workpiece coating.

While extended periods of commercially acceptable productions have beenattained by this method, considerable difficulty is often encounteredwhen a series of aluminum workpieces are successively coated. It hasbeen observed that the quality of the coating deposited on theindividual workpieces suffers as the series progresses. For instance,the quality of the coating deposited after one month of operation areoften not as smooth, uniform and glossy as those coatings depositedinitially. Additionally, it has been observed that the coating color isoften off specification after prolonged operation. This is particularlytrue in the case of white and other light colored coatings.

I have found that this deterioration in coating quality is due primarilyto an accumulation of inorganic, anionic contaminants in the coatingbath. These anionic contaminants are primarily inorganic mineral acidanions such as sulfates acid sulfates (HSOg), chlorides c1 fluorides(F), bromides (BI nitrates (NO nitrites (N05) carbonates (CO acidcarbonates (HCO phosphates (PO phosphites (PO chromates (CrOE) andsulfites (80 principally halides and oxygenated anions of elementshaving atomic numbers between 6 and 25 inclusive.

In spite of efforts to prevent these contaminants from entering thebath, they are often introduced in the form of improperly deionizedwater, or by improperly cleaned workpieces. This is particularly true incommercial operations where thorough deionization of large workpieces isnot practical.

The inorganic anion concentration in the bath that causes a detrimentaleffect is quite small as compared to the concentration of thepolycarboxylic resin, in that rough, heavy, non-uniform electrocoatingsare deposited on aluminum substrates when the total concentration of theabove and other inorganic acid anions exceeds about 30 parts per million(p.p.m.). According to the present invention, the total inorganic anionconcentration is maintained below 10 p.p.m. and preferably below 5p.p.m. through an ion exchange process.

The detrimental effects of the individual inorganic anions are additivein approximate proportion to their respective concentrations in p.p.m.For example, a concentration of 15 p.p.m. of sulfate (S05) and 20 p.p.m.nitrate (NO can produce an unsatisfactory coating, while coatingsdeposited in a bath containing either anion alone would be satisfactory.

Since anions will exert electrolytic influence in proportion to thenumber of ionic equivalents that are present, there is no theoreticalbasis for using the concentration in p.p.m. of chemically differentanions as a common basis of comparison. This is just a convenient,empirical relationship of practical significance.

In practicing the present invention, the total inorganic anionconcentration is maintained at a below 30 p.p.m. (usually less than 10p.p.m.) by contacting the electrocoat bath with an anion exchange mediumcapable of exchanging an innocuous ion for inorganic anioniccontaminants in the bath, without substantially removing the anionicallydispersed polycarboxylic coating resin from the bath. These requirementscan 'be fulfilled by synthetic anion exchange resins in the hydroxylform. An anion exchange resin is in the hydroxyl form when it hashydroxyl ions (OH) available for external exchange.

Ion exchange is a phenomenon whereby a highly active ion associated witha relatively inactive nucleus (ion exchange resin matrix) can beexchanged for another active but different ion in solution. Ion exchangeresins are well known in the art and can be visualized as having anelastic, three dimensional hydrocarbon network containing ion activegroups. The network is prepared from a copolymer of styrene anddivinylbenzene, the latter serving as a cross-linking agent and also toform a three dimensional structure. Additional information on ionexchange resins is given in US. Pats. 2,366,007; 2,341,907; 2,591,574;2,591,573 and 2,614,099; which are incorporated herein by reference.

Synthetic anion exchange resins are cross-linked polyelectrolytescomprising a polymeric matrix containing a large number of ion activegroups. The polymeric matrix is dimensionally stable, porous, and inert.Cationic active groups are firmly attached in this polymeric matrix, andare immobile. The electrical charge of these immobile cationic groups isbalanced by an equivalent number of anionic groups (e.g. hydroxyl) whichare mobile and can exchange with other ions of similar charge from anexternal source. When the anionic exchange resin has hydroxyl groups(OH-) as the mobile exchangeable group, the anionic exchange resin issaid to be in the hydroxyl form.

According to the present ion exchange process, harmless hydroxyl ionenter the bath while the detrimental inorganic acid anions are removed.

In selecting particular anion exchange resins, pore size of thepolymeric matrix is selected to be large enough to admit the inorganicanionic contaminants from the bath, but small enough to exclude thelarger anionic polycarboxylic coating resin. Suitable commerciallyavailable anion exchange resins include Dowex l, Dowex 2, Dowex 11 andDowex 21K resins in the hydroxyl form. These resins are strong baseanion exchange resins incorporating quaternary ammonium functionalityand are sold by the Dow Chemical Company. Other suitable resins includethe hydroxyl form of the IRA series of the Macroreticular Amberlite IonExchange Resins sold by the Rohm and Haas Company.

The anion exchange resin can be contacted with the contaminatedelectrocoat bath by any conventional technique. For instance, a quantityof the anion exchange resin can be mixed in the bath and then removedafter the inorganic acid anion concentration is measured to be below theacceptable level. Alternatively, the ion exchange can be accomplished bycirculating (either periodically or continuously) the bath through anexternal bed of the ion exchange resin. External circulation through aresin bed is preferred in the interest of commercial practicality.

The reasons that these anions interfere with the coat ing quality is notpresently understood, although it is strongly suspected that theseanions, being quite mobile electrically, compete with the polycarboxylicresin anions in collecting at the anode. These inorganic acid anionsthen deposit on, and chemically react with, the aluminum workpiecetogether with the coating film. As the coating builds up, these anionsbecome entrapped on the aluminum surface and encapsulated within thecoating film. This results in the deposition of a heavy, rough coatingof nonuniform character. The coating also tends to be darkened by thepresence of these anions. This is particularly undesirable when white orother light colored coatings are being deposited.

Besides having a detrimental effect on the quality of the coating asdeposited, the anions entrapped in the coating film also provide latentchemical reaction sites which can be activated during the service lifeof the coated workpiece. This is particularly true in the case ofaluminum in that bubbles and blisters often develop in electrocoatedaluminum articles. This is probably caused by the high chemicalreactivity of aluminum. For instance, an aluminum workpiece that hasbeen coated in an electrocoating bath containing excessive mineral acidanion contaminants will be more sensitive to ordinary solvents andalkaline cleaning solutions than an aluminum workpiece that has beencoated in a similar electrocoating bath that is free from suchcontaminants.

The following exampes show how the invention can be practiced, butshould not be construed as limiting the invention. All parts are partsby weight and all percentages are weight percentages unless otherwiseindicated.

EXAMPLE 1 An acrylic resin is made by slowly adding a mixture of 60parts of butyl acrylate, 25 parts of styrene, 15 parts of methacrylicacid, and 2 parts of dicumyl peroxide into 23 parts of 2-butoxy ethanol.This reaction mixture is maintained at about 158 to 170 C. for a fourhour period in an agitated reactor equipped with a reflux condenser. Theresulting reaction product is cooled to about 100 C., and 18 parts ofhexakis (methoxy methyl) melamine (Cymel 300, sold by American CyanamidChemical Company) is added over a 15-minute period. The re- 8 sultingacrylic resin dispersion is then allowed to cool to room temperature.This acrylic resin dispersion apparently soluble in the electrocoatingbath hereinafter described.

A white paint concentrate, containing about 35% NVM (non-volatilematerials) is prepared by blending 260 parts of the acrylic resindispersion prepared above, 17 parts of mineral spirits, 44 parts ofdiisopropanolamine, 442 parts of deionized water, and 122 parts of apigment grind. The pigment grind is prepared by mixing 92 parts of theacrylic resin dispersion prepared above, 18 parts of diisopropanolamineand 453 parts of deionized water with 387 parts of kaolin clay and 580parts of pigment grade TiO powder in a pebble mill.

The electrocoating bath is then prepared by diluting the above describedwhite paint concentrate with deionized water to 8% NVM with deionizedwater. The deionized water has a specific resistance greater than 50,000ohm-cm. at F.

The term NVM has been used above. This term refers to non-volatilematerial and is determined according to the ASTM test for Non-volatileContent of Varnishes, test designation D-l644-59.

The anodes used are 10.16 cm. wide by 8.89 cm. deposit length sheetaluminum panels, and the painting operation is conducted in a metal tankequipped with an agitator. The tank is wired as the cathode. The tankcontains about 1100 mls. of the electrocoating bath described above, ata temperature of about 30 C. A constant D.C. potential of about volts isimpressed across the tank cathode and the aluminum panel anode, from anexternal circuit. The anode is then slowly immersed in the bath over a15-second period, and electrical current flows between the anode andcathode, while the D.C. potential is maintained constant at about 150volts. After one minute of electropainting under these conditions(including the 15-second immersion period), a coating is deposited onthe aluminum anode. This electrodeposited paint film is white in color,water resistant, slightly tacky, and tenaciously adherent. After ovendrying at about 176 C. for about 15-30 minutes, the coating is about 1mil in thickness, and is tough, uniform and quite glossy.

EXAMPLE 2 The electrocoating bath of Example 1 is contaminated withabout 75 parts per million (ppm) of sulfate ion by adding sulfuric acidto the bath. This condition is designed to simulate a commercial coatingoperation wherein mineral acid anionic contaminants enter the bath withcommercial aluminum parts and/or improperly deionized water.

The coating operation is conducted as in Example 1. The aluminum panels,after coating, but before baking, have a very rough, uneven texture andare off-white in appearance. After baking, the coatings have a slightyellowish cast and are very rough and non-uniform in appearance and intexture. Additionally, the coatings are very low in gloss.

To further demonstrate the detrimental effects of inorganic acid anioniccontaminants in the electrocoating bath, several additional experimentsare performed using the method of Example 1. In these experiments, thevoltage is held constant at the level indicated in each run, and thecoating time is constant in each run at one minute. As in Example 1, theanode panel is immersed in the electrocoating bath at a uniform raterequiring about 15 seconds for the panel to be immersed to the 8.89 cm.deposit length. This entrance time is included in the one minute coatingperiod.

The following table presents the thickness of the coating deposited inmils, as a function of the constant D.C. voltage impressed across theelectrodes in an anionically contaminated electrocoating bath.

The gloss of the coating deposited (after oven drying) is evaluated by areflectance gloss meter test method wherein light beams strike thesurface at a given angle and the light beams reflected from the surfaceare detected and measured by a photocell. The test method used is theASTM test D-523-62-T, entitled Specular Gloss. In this test method, thehigher test values indicate higher gloss. The angle of the incident beamis 60. The test results are set forth below:

Thickness 60 of coating gloss Constant voltage between deposited toanode and cathode (mils) value EXAMPLE 3 The sulfate contaminatedelectrocoating bath of Example 2 is circulated through a bed of ionexchange resin until the sulfate ion level is reduced to about 5-10p.p.m. The bed is a /2 inch diameter by 36 inch glass tube containingabout 120 mls. of anion exchange resin. The resin used is Dowex 2X8 ionexchange resin in the hydroxyl form. The flow rate of the electrocoatingbath through the bed is about 10 mls. per minute.

Aluminum panels are coated by the method of Example 2 in the ionexchanged electrocoating bath of this example. The coated panels arethen evaluated by the method of Example 2. The results are set forthbelow:

Thickness 60 of coating gloss Constant voltage between deposited testanode and cathode (mils) value EXAMPLE 4 The bath of Example 1 iscontaminated with about 50 p.p.m. of nitrate ion by adding nitric acidto the bath. This example demonstrates the detrimental effect of thepresence of the nitrate ion. The nitrate ion is a common contaminant incommercial processes. Aluminum panels are coated by the method ofExample 2 and thick, uneven, rough yellowish coatings are obtained. Thephysical characteristics of these coatings are set forth below.

Thickness of coating deposited (mils) Constant voltage between anode andcathode Appearance Smooth.

0 Rough.

3 Very rough. g D

0. Extremely rough.

EXAMPLE 5 The coating bath of Example 4 is circuited through an ionexchange resin bed similar to the bed described Thickness of coatingdeposited (mils) Constant voltage between anode and cathode AppearanceVery smooth. 0.

These panels are very white, very glossy and have a smooth, uniformsurface texture.

These coated panels are superior to the panels prepared in Example 4 asis evident by comparing the physical data.

While the invention has been described in exemplary detail with respectto sulfate and nitrate contaminants, the present method will improve theperformance of electrocoating baths that contain any inorganic acidanionic contaminants.

By aluminum anode or workpiece I mean to include those having surfacesof aluminum, anodized aluminum, and alloys which are preponderantlyaluminum, eg about to 99+% aluminum.

Having thus described the invention, what is claimed is:

1. In the method for electrically depositing a coating on a series ofaluminum articles wherein each of said articles successively serves asan anode in an electrical circuit while in contact with anelectrocoating bath, said bath comprising an aqueous anionic dispersionof a polycarboxylic, film-forming resin, said bath further containinginorganic acid anions as impurities; and an electric current flowsbetween said anode and a cathode in electriacl communication with saidbath under sufficient electrical potential to deposit a coating of saidfilm-forming resin upon said anode from said bath, the improvement whichcomprises:

maintaining the inorganic acid anion concentration of saidelectrocoating bath below about 30 p.p.m. by contacting said bath withan anion exchange resin in the hydroxyl form, whereby the depositedcoating properties of uniformity and glossiness are enhanced.

2. The method of claim 1 wherein said bath is passed through a bed ofsaid anion exchange resin.

3. The method of claim 1 wherein said ion exchange resin is mixed in theelectrocoating bath and then removed from said bath when said inorganicacid anion concentration is below about 30 p.p.m.

4. The method of claim 1 wherein the inorganic acid anion concentrationis maintained below about 10 p.p.m.

5. In a method for electrocoating a series of aluminum articles whereineach of said articles is successively contacted With an aqueous coatingbath having paint dispersed therein and a cathode in electricalcommunication therewith, said aluminum article serving as an anode,wherein mineral acid anions are present as impurities in said bathduring the electrocoating process, said paint containing a predeterminedfraction of the film-forming paint binder a synthetic polycarboxylicacid resin at least partially neutralized with a suflicient quantity ofa water soluble amino compound to maintain said polycarboxylic acidresin as a dispersion of anionic polyelectrolyte in said bath, said acidresin having electrical equivalent weight between about 500 and about20,000, acid number between about 20 and about 300 and said resinexhibiting anionic polyelectrolyte behavior in said bath as indicated byits depositing as a coating on said anode substantially directlyproportionally to electric current flow through said bath under theinfluence of an electric potential between said 1 1 1 2 cathode and saidanode, the improvement which com- References Cited prises: NT

contacting said bath with an ion exchange resin in the UNITED STATESFATE 5 hydroxyl form to maintain the mineral acid anion 3,355,37311/1967 Brewer a1 204 181 concentration of said bath below about 30p.p.rn. 5 3,444,064 5/1969 Johnson wheregy the deposited colatinggroperties of uniform- FOREIGN PA 1ty an g ossmess are en ance 6. Themethod of claim 5 wherein said bath is passed ggiggg i zfi g through abed of said anion exchange resin.

7. The method of claim 6 wherein said mineral acid 10 1504209 10/1967France 204 181 anion concentration is maintained below about 10 ppm.HOWARD S, WILLIAMS, Primary Examiner

